Various earth-boring tools such as rotary drill bits (including roller cone bits and fixed-cutter or drag bits), core bits, eccentric bits, bi-center bits, reamers, and mills are commonly used in forming boreholes or wells in earth formations. Such tools often may include one or more cutting elements on a formation-engaging surface thereof for removing formation material as the earth-boring tool is rotated or otherwise moved within the borehole.
For example, fixed-cutter bits (often referred to as “drag” bits) have a plurality of cutting elements affixed or otherwise secured to a face (i.e., a formation-engaging surface) of a bit body. Cutting elements generally include a cutting surface, where the cutting surface is usually formed out of a superabrasive material, such as mutually bound particles of polycrystalline diamond. The cutting surface is generally formed on and bonded to a supporting substrate of a hard material such as cemented tungsten carbide. During a drilling operation, a portion of a cutting edge, which is at least partially defined by the peripheral portion of the cutting surface, is pressed into the formation. As the earth-boring tool moves relative to the formation, the cutting element is dragged across the surface of the formation and the cutting edge of the cutting surface shears away formation material. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutting elements, or cutters.
During drilling, cutting elements are subjected to high temperatures due to friction between the cutting surface and the formation being cut, high axial loads from the weight on bit (WOB), and high impact forces attributable to variations in WOB, formation irregularities and material differences, and vibration. These conditions can result in damage to the cutting surface (e.g., chipping, spalling). Such damage often occurs at or near the cutting edge of the cutting surface and is caused, at least in part, by the high impact forces that occur during drilling. Damage to the cutting element results in decreased cutting efficiency of the cutting element. When the efficiency of the cutting element decreases to a critical level the operation must be stopped to remove and replace the drill bit or damaged cutters, which is a large expense for an operation utilizing earth-boring tools.
Securing a PDC cutting element to a drill bit restricts the useful life of such cutting element, as the cutting edge of the diamond table wears down as does the substrate, creating a so-called “wear flat” and necessitating increased weight on bit to maintain a given rate of penetration of the drill bit into the formation due to the increased surface area presented. In addition, unless the cutting element is heated to remove it from the bit and then rebrazed with an unworn portion of the cutting edge presented for engaging a formation, more than half of the cutting element is never used.
Attempts have been made to configure cutting elements to rotate such that the entire cutting edge extending around each cutting element may selectively engage with and remove material. By utilizing the entire cutting edge, the effective life of the cutting element may be increased. Some designs for rotating cutting elements allow the cutting element to freely rotate even when under a cutting load. Rotating under a load results in wear on internal surfaces, exposing the cutting element to vibration, which can damage the cutting elements reducing their life, and may result in uneven wear on the cutting edge of the cutting element.